Reverse osmosis support substrate and method for its manufacture

ABSTRACT

A nonwoven composite web is formed by a wet process on a papermaking machine. The web coming off the papermaking machine is dried and thermally bonded using heated calendar rolls. The nonwoven composite material is made from a furnish of polymeric staple fibers, a first binder fiber consisting, at least in part, of a first thermoplastic binder material which melts at a first melting temperature less than and a second binder fiber consisting, at least in part, of a second polymeric material which has second melting temperature which is higher than the first melting temperature. The first polymeric material is selected to have a first melting temperature less than the temperature to which the first material will be subjected in the papermaking machine. The melted first polymeric material gives the web strength on the papermaking machine. The second polymeric material is selected to have a second melting temperature less than the temperature to which the second material will be subjected in the calendar rolls. Thus, the second polymeric material is melted as the web passes through the calendar rolls and thermally bonds the other fibers of the web when the melted second polymeric material fuses upon cooling. The staple fibers are made of polyester and the bicomponent binder fibers are of the co-polyester/polyester sheath/core variety. The support substrate has a sheet porosity in the range of 5-10 cfm.

FIELD OF THE INVENTION

This invention generally relates to synthetic nonwoven materialsfabricated by wet-laid processes. In particular, the invention relatesto a paper-like web made with polyester fibers which is useful as asupport substrate for a reverse osmosis membrane.

BACKGROUND OF THE INVENTION

Reverse osmosis is a process for purification of saline water. Inaccordance with this process, a pressure in excess of the osmoticpressure of the saline water feed solution is applied to the feedsolution to separate purified water using a semipermeable membrane.Purified water diffuses through the membrane while salts and otherimpurities are retained by the membrane.

The mechanical properties of the membranes often require reinforcementor support, for example, with polyester woven or nonwoven fabric, inorder to prepare satisfactory reverse osmosis structures. Typically, areverse osmosis membrane is formed by coating or casting an appropriatepolymer (e.g., polysulfone) solution onto the supporting fabric. Withthe coating of extremely thin membranes, continuity of the membrane onthe supporting surface is difficult to obtain. Moreover, the lack ofproper adhesion between the supporting fabric and the reverse osmosismembrane can lead to delamination in use, which results in the formationof blisters between the supporting fabric and the reverse osmosismembrane.

Fabrics used as a support substrate in reverse osmosis applications arepreferably low in cost, stable under the operating conditionsencountered in reverse osmosis operations and highly porous so as toretain high flux in operation.

A support material disclosed in U.S. Pat. No. 4,454,176 to Buckfelder isprepared by weaving a yarn of polyester. The woven cloth is dried, heatset and calendared, and used as the reverse osmosis membrane substrate.Other support fabrics used by Buckfelder to make exemplary supportedreverse osmosis membranes include a calendared spunbonded polyesterfabric and a resin-bonded polyester fabric.

U.S. Pat. No. 4,728,394 to Shinjou discloses a semipermeable membranesupport of the type which is used for precise filtrations such as inultrafiltration and reverse osmosis. Such precise filtration is utilizedin desalination of seawater, the food industry, treatment of industrialwastewater and medical applications. The Shinjou '1394 patent disclosesthat semipermeable membranes comprising synthetic polymer, without anysupport, are inferior in mechanical strength. Therefore such membranesare generally used with a backing material such as woven fabric. Forexample, reverse osmosis membranes are formed by casting a polymersolution directly onto a porous support such as a nonwoven fabric.Conventional membrane support substrates include woven and knittedfabrics, nonwoven fabric, porous sintered material, or paper.

In particular, various support substrates which use nonwoven fabric havebeen developed. However, supports having a high density causeinsufficient penetration of the polymer solution. A consequence is thedelamination between layers due to reduced peeling strength between thesupport and membrane. Another consequence can be the generation ofpinholes due to residual bubbles, because of insufficient debubbling inthe support. Low-density support substrates, in spite of sufficientpenetration by the polymer solution, result in over penetration of thepolymer solution to the back surface opposite to the casted surface. Theresulting membranes give uneven filtration. Severe defects such asreduced filtration performance and/or damage of the semipermeablemembrane due to the partial excessive pressurization during filtrationoperation can result.

The Shinjou '394 patent proposes to solve the foregoing problems byforming a low-density nonwoven layer to a high-density nonwoven layerand laminating the layers using a heated calendar. Then a polymersolution is cast on the low-density layer of the laminated support. Thelow-density nonwoven layer is formed by a dry process and thehigh-density nonwoven layer is formed by a wet process. All of thefibers in both layers are polyester. The dry-processed polyesternonwoven layer comprises 20-80%. preferably 30-60%, binder fibers, whichencompasses both undrawn polyester fibers and conjugate (i.e.,bicomponent) polyester fibers. The wet-processed nonwoven layer isformed entirely of polyester fibers having a denier of 1.5 or less,comprising 30-90%, preferably 40-70%, undrawn or conjugate polyesterfibers. The wet process is a conventional papermaking process followedby heated calendaring. In the examples of Shinjou, the wet-processednonwoven consisted of 50% polyester staple fibers having a denier of 1.0and a length of 5 mm and 50% of undrawn polyester fibers having a denierof 1.0 and a length of 5 mm.

The foregoing prior art does not disclose a nonwoven support substratemade by a wet process on a papermaking machine without lamination to adry-processed nonwoven web.

SUMMARY OF THE INVENTION

The present invention is a nonwoven support substrate which is formed bya wet process on a papermaking machine without lamination to adry-processed nonwoven web. The web coming off the papermaking machineis dried and thermally bonded using heated calendar rolls. Thisinvention has the benefit of eliminating the manufacturing costsassociated with dry web formation.

The nonwoven support substrate in accordance with the preferredembodiment of the invention is a composite material comprising polymericstaple fibers, a first binder fiber consisting, at least in part, of afirst thermoplastic binder material which melts at a first meltingtemperature less than the melting temperature of the polymeric staplefibers, and a second binder fiber consisting, at least in part, of asecond thermoplastic binder material which melts at a second meltingtemperature less than the first melting temperature. The firstthermoplastic binder material is selected to melt at the temperaturewhich it is exposed to in the calendar during thermal bonding. Thesecond thermoplastic binder material is selected to melt at thetemperature which it is exposed to in the papermaking machine. Themelted second thermoplastic binder material gives the web strength onthe papermaking machine.

In accordance with the preferred embodiment of the fiber furnish, thestaple fibers are made of polyester and the bicomponent binder fibersare of the sheath/core variety. The lower-melting-point bicomponentbinder fiber has a co-polyester sheath and a polyester core. Theco-polyester sheath melts at a temperature of 225° F. Thehigher-melting-point bicomponent binder fiber also has a co-polyestersheath and a polyester core, but the co-polyester sheath melts at atemperature of 375° F.

An important feature of a membrane support substrate is its sheetporosity. For example, in the case where the polymer cast onto thesupport substrate is polysulfone, if the sheet porosity is too low, thepolysulfone will not attach to the support substrate. On the other hand,if the sheet porosity is too high, the polysulfone penetrates thesupport substrate too much and does not form a film on the surface. Inaccordance with the present invention, the fiber deniers and lengths areselected to achieve a sheet porosity in the range of 5-10 cfm. Inaccordance with the present invention, this is achieved by usingpolyester staple fibers having a denier in the range of 0.2 to 3.0.

Alternatively, the binder fiber contains thermoplastic material having amelting temperature different than that of the polyester staple fibers,but the binder fiber need not be bicomponent. The thermoplastic materialof such binder fibers can be a polymer different than polyester, e.g.,polyethylene, or a polyester having a molecular weight which isdifferent than the molecular weight of the polyester staple fibers.

The component fibers are combined with water into a homogeneous mixtureand formed into a mat employing a wet-lay process. A high strengthpaper-like material is formed by thermally bonding the mat undercontrolled temperature and pressure conditions.

Strength and porous characteristics are imparted to the composite by thecombination of polyester fibers employed in the invention. Inparticular, the strength of the composite can be improved by varying thepolyester fiber content in accordance with the following functionalrelations: (a) as the polyester denier increases at constant length andamount, the porosity, bulk and stiffness of the composite increase andthe amount of fiber entanglement decreases; (b) as the polyester lengthincreases at constant denier and amount, the tensile and tear strengthsin the MD and CD directions and the Mullen burst strength increase andthe stiffness decreases; and (c) as the quantity of polyester increasesat constant denier and length, the tensile strength improves, Mullenburst and tear strengths, and porosity increase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of an apparatus for preparation of stockor furnish for manufacture of the composite material of the invention.

FIG. 2 is a diagrammatic view of an apparatus for formation and dryingof a web employed in the manufacture of the composite material.

FIG. 3 is a diagrammatic view of an apparatus for thermally bonding theweb to form the composite material of the invention.

FIG. 4 is a diagrammatic view of an alternative calendar which can beincorporated in the thermal bonding apparatus shown in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the preferred embodiments of the invention, the fiberfurnish comprises 5 to 40 wt. % of a first polyester staple fiber havinga first denier in the range of 0.2 to 3.0; 0 to 60 wt. % of a secondpolyester staple fiber having a second denier greater than the firstdenier, but also in the range of 0.2 to 3.0; 15 to 50 wt. % of a firstbinder fiber incorporating material having a first melting point whichis lower than the melting point of the polyester of the staple fibers;and 1 to 10 wt. % of a second binder fiber incorporating material havinga second melting point which is lower than the first melting point.

In accordance with the most preferred embodiment, the fiber furnishconsists of 20.4 wt. % 0.43-denier×10 mm polyester staple fiberssupplied by Kuraray Co., Ltd., Osaka, Japan (hereinafter "Kuraray")(melting temp. 480° F.); 36.6 wt. % of a bicomponent fiber having a coremade of polyester and a sheath made of a co-polyester which melts at afirst melting temperature less than the melting temperature of thepolyester staple fibers or the melting temperature of the polyestercore, namely, 1.0 denier×5 mm EP-101 bicomponent binder fiber suppliedby Kuraray (melting temp. 375° F.); 2.3 wt. % of a bicomponent fiberhaving a core made of polyester and a sheath made of a co-polyesterwhich melts at a second melting temperature less than the first meltingtemperature, namely, 2.0 denier×5 mm Type N-720H bicomponent binderfiber supplied by Kuraray (melting temp. 225° F.); and 40.7 wt. %1.5-denier×1/2" Type 108 polyester staple fibers supplied byHoechst/Celanese (melting temp. 480° F.). All of the foregoing fibertypes are sized by the respective manufacturer.

A high-strength nonwoven material is formed by a wet-laying process on aconventional papermaking machine. Then the nonwoven material isthermally bonded under controlled temperature and pressure conditions.In accordance with the method of the invention, a wet-laid mat of thecomposite material is dried at temperatures in the range of 200°-285° F.and then thermally calendared with rolls heated to temperatures of 425°F. and nip pressures of 50 psi or greater. The weight per unit area ofthe composite following thermal calendaring can be varied from 25 to 55pounds per 3000 ft² depending on the sheet composition and thecalendaring conditions chosen to effect a certain set of physicalproperties.

In accordance with further variations of the invention, the length ofthe small-denier polyester staple fibers and the lengths of theco-polyester/polyester bicomponent fibers can be varied between 5 and 15mm, while the length of the large-denier polyester staple fibers can bevaried between 1/8" and 11/2".

Also, 1/2"×3.0-denier polyester binder fiber, such as Type 259 suppliedby Hoechst Celanese Corporation, Wilmington, Del., may be added orsubstituted for some of the bicomponent binder fibers.

FIG. 1 illustrates an apparatus for preparation of stock or furnish formanufacture of the composite in accordance with the preferredembodiment. A batch of polyester is prepared in a hydropulper 10, whichcontains water. In preparation of the slurry, the water is agitated, asurfactant (Pluronic F-108 supplied by BASF Corporation) is added to aconcentration of 0.5% based on fiber weight and the polyester staplefibers and co-polyester/polyester bicomponent binder fibers areintroduced into the furnish in the following sequence:

(1) Kuraray N-720H bicomponent binder fibers;

(2) Kuraray EP-101 bicomponent binder fibers;

(3) Kuraray 0.43-denier polyester fibers; and

(4) Hoechst/Celanese Type 108 polyester fibers. The volume of water andamount of fiber is such that the consistency of the furnish in thehydropulper is about 3% solids. After all of the fibers have been addedto the furnish, the furnish is mixed for approximately 3 minutes todisperse the polyester and bicomponent fibers. A web formation aid,e.g., an anionic polyacrylamide, is added to the furnish to aconcentration of 1-2% based on fiber weight (Reten 235 supplied byHercules Inc.). The slurry is mixed for a sufficient time to dispersethe polyester fibers in a uniform fashion. Visual inspection is used todetermine when the fibers are totally separated and well dispersed. Thefiber slurry is then transported to mixing chest 14 via valve 12. Inmixing chest 14 the polyester slurry is diluted to the desiredconsistency, i.e., approximately 1% solids.

After the polyester slurry has been suitably mixed in mixing chest 14,the slurry is transported via opened valve 16 to the machine chest 18,where the slurry is further diluted to a consistency of approximately0.6% solids. Thereafter, the slurry is transported to the web-formingmachine via valve 20.

FIG. 2 is a diagrammatic view of an apparatus for formation and dryingof a web employed in the manufacture of the composite in accordance withthe invention. The homogeneous fiber slurry is received by headbox 26.In the headbox, the slurry has a consistency of about 0.05% solids. Aweb 32 is formed by machine 28 using a wet-lay process in accordancewith conventional papermaking techniques. The temperature which thefibers are exposed to on the wet-laying machine lies in the range of325°-365° F. During the wet-laying process, the co-polyester sheathmaterials of the N-720H bicomponent binder fibers (which sheath materialhas a melting point of 225° F.) melts and then fuses upon cooling tolend strength to the web during further processing. Thereafter, the web32 enters a stack of drying rollers 30, which remove water from the web.The dried web 32 is then wound up on a reel (not shown in FIG. 2) forfurther processing.

A high-strength and densified composite material is provided bythermally bonding the dried web 32 in a calendar, as shown in FIG. 3. Onthe process line, the web 32 is unwound from the reel 34, and fed byguide roll 36 to the nip between calendar rolls 38 and 38'. Calendarrolls 38 and 38', which are preferably fabricated of steel, are heatedto a temperature and maintained at a compression pressure in the rangeof 385°-435° F. and of 400-1100 pli. Preferred results are obtained at atemperature of approximately 425° F. and pressure of 800 pli.

In the alternative, the web can be partially wrapped around a roll 39which is heated to a temperature of about 385° F. and then passedbetween the calendar rolls 38 and 38' in an S-configuration, as seen inFIG. 4. The heated roll 39 preheats the web before it enters thecalendaring roll nip. Preheating allows a faster speed of the productionline.

After thermal bonding in the calendar rolls, the web in successionenters a second nip formed by a soft top roll 40 and a steel bottom roll42 and a third nip formed by a steel top roll 44 and a soft bottom roll46. The pressure at the second and third nips is 15 to 35psi. Afterpassing between rolls 44 and 46, the thermally bonded web contacts guideroll 48 and is then wound up on a reel 50.

Table 1 sets forth physical properties of the preferred embodiment ofthe invention both before and after thermal bonding.

                  TABLE 1                                                         ______________________________________                                        Physical Properties of Composite Material                                     TAPPI* No.                                                                            Physical Property                                                                             Uncalendared                                                                             Calendared                                 ______________________________________                                        410     Basis Weight    46.0       46.0                                               (3000 ft.sup.2)                                                       411     Caliper (mils)  22.0       4.0                                        251     Porosity-Permeability,                                                                        240        5-10                                               Frazier Air (cfm)                                                     403     Mullen Burst (psi)                                                                            5          136                                        414     Elmendorf Tear (gm)                                                                           10/15      143/155                                            (MD/CD)                                                               494     Instron Tensile (lb/in.)                                                                      1.8/1.2    35.0/10.0                                          (MD/CD)                                                               494     Elongation (%) (MD/CD)                                                                        2.4/4.5    4.0/5.0                                    ______________________________________                                         *Standard of the Technical Association of the Pulp and Paper Industry         ("TAPPI"), Technology Park, Atlanta, Georgia.                            

The calendared composite exhibits a microstructure in which fiberinterfaces are fused due to melting of the co-polyester sheaths of theEP-101 bicomponent binder fiber. The co-polyester sheath has a meltingpoint lower than that of the polyester core or the polyester staplefibers. The calendaring of the composite web effects a reduction in thefiber spacing, i.e., by fiber compression and bonding. The density ofthe web material and the flatness (levelness) of the surface of the webmaterial are substantially enhanced in the calendaring process.

The foregoing preferred embodiments have been described for the purposeof illustration only and are not intended to limit the scope of theclaims hereinafter. Variations and modifications of the composition andmethod of manufacture may be devised which are nevertheless within thescope and spirit of the invention as defined in the claims appendedhereto. For examples, it will be apparent to practitioners of ordinaryskill that binder fibers different than those specified herein may beused, so long as the binder fiber contains thermoplastic material havinga melting point lower than that of the polyester fibers and providingadequate bonding of those polyester fibers to form a nonwoven web withhigh tensile strength. In addition, polyester staple fibers of 0.2 to3.0 denier can be used and blended in various ratios to effect desiredphysical properties. The range and blend of bicomponent binder fibersmay also be varied to effect desired physical properties. Furthermore,the physical properties as well as the performance of the sheet materialcan be altered to fit a particular set of physical specifications byadjusting the furnish composition and ratio as well as the calendaringparameters. Sheet basis weights may also vary from 25.0 to 55.0 lb.(3,000 ft² basis) depending on the sheet fiber composition and thecalendaring conditions chosen to effect a certain set of physicalproperties.

The final sheet material in accordance with the preferred embodiments ofthe invention is suitable for use as the support substrate in a reverseosmosis membrane. The reverse osmosis membrane is formed by casting athin film (2-4 mils thick) of polymeric material, preferablypolysulfone, on one surface of the nonwoven support substrate in thewell-known conventional manner (see, e.g., U.S. Pat. Nos. 4,454,176,4,728,394 and 5,028,329, the disclosures of which are specificallyincorporated by reference herein). It is important that all surfacefibers of the nonwoven support substrate be tied down by fused bindermaterial so that the surface fibers do not poke a hole in thepolysulfone film. Also the sheet porosity of the nonwoven supportsubstrate should be in the range of 5-10 cfm to ensure that thepolysulfone attaches securely to the surface without penetrating deeplyinto the substrate.

I claim:
 1. A method of manufacturing a nonwoven web made from a furnishcomprising:5 to 40 wt. % of polymer staple fibers having a deniergreater than 0.2 but less than 1.0; 0to 60 wt. % of polymer staplefibers having a denier greater than 1.0 but less than 3.0; 15 to 50 wt.% of co-polymer/polymer bicomponent fibers having a first co-polymersheath material which melts at a first melting temperature less than themelting temperature of said polymeric staple fibers; and 1 to 10 wt. %of co-polymer/polymer bicomponent fibers having a second co-polymersheath material which melts at a second melting temperature less thanthe first melting temperature, comprising the steps of:wet-laying a matof fibers from said furnish using a papermaking machine; drying saidwet-laid mat at a drying temperature greater than said second meltingtemperature and less than said first melting temperature; andcalendaring said web at a calendering temperature greater than saidfirst melting temperature and less than said melting temperature of saidpolymeric staple fibers.
 2. The method as defined in claim 1, whereinsaid polymeric staple fibers are made of polyester.
 3. The method asdefined in claim 1, wherein said first co-polymer sheath material isco-polyester.
 4. The method as defined in claim 1, wherein saidco-polymer/polymer bicomponent fibers comprise co-polyester/polyesterbicomponent fibers.
 5. A method of manufacturing a nonwoven compositeweb comprising the following steps:forming a furnish by mixing 40 to 84wt. % of polymeric staple fibers having a length in the range of 5 to 15mm and a denier in the range of 0.2 to 3.0, and 16 to 60 wt. % ofpolymeric binder fibers, wherein a first fraction of said polymericbinder fibers comprise a first thermoplastic binder material having afirst melting temperature less than the melting temperature of saidpolymeric staple fibers and a second fraction of said polymeric binderfibers comprise a second thermoplastic binder material having a secondmelting temperature less than said first melting temperature; wet-layinga mat of fibers from said furnish using a papermaking machine; dryingsaid wet-laid mat at a drying temperature greater than said secondmelting temperature and less than said first melting temperature; andcalendaring said mat alone at a calendering temperature in excess ofsaid first melting temperature but less than said melting temperature ofsaid polymeric staple fibers.
 6. The method as defined in claim 5,wherein said drying temperature is in the range of 200° to 285° F. andsaid calendering temperature is in the range of 385° to 435° F.
 7. Themethod as defined in claim 5, wherein said first thermoplastic bindermaterial is a co-polyester.
 8. The method as defined in claim 5, whereinsaid first thermoplastic binder material is a co-polyester from a firstfraction of co-polyester/polyester bicomponent fibers and said secondthermoplastic binder material is a co-polyester from a second fractionof co-polyester/polyester bicomponent fibers.
 9. The method as definedin claim 5, wherein said first thermoplastic binder material ispolyethylene.
 10. The method as defined in claim 5, wherein said firstthermoplastic binder material is polyester having a molecular weightdifferent than the molecular weight of said polyester staple fibers. 11.The method as defined in claim 5, wherein said polymeric staple fiberscomprise a first fraction of polyester staple fibers having a deniergreater than 0.2 but less than 1.0 and a second fraction of polyesterstaple fibers having a denier greater than 1.0 but less than 3.0. 12.The method as defined in claim 5, wherein said furnish comprises 5 to 40wt. % of said polyester staple fibers of said first fraction and 0 to 60wt. % of said polyester staple fibers of said second fraction.